A PET bottle obtained by molding polyethylene terephthalate (hereinafter abbreviated as “PET”) is excellent in not only physical properties such as strength, transparency and gloss, but also hygienic property and safety.
As processes for molding the PET bottle, are generally adopted a direct blow molding process and a stretch blow molding process. The direct blow molding process is a process in which a parison in a molten state is formed by means of an extruder or injection molding machine, and the parison is subjected to blow molding within a blow mold. Many of PET bottles are produced by the stretch blow molding process. According to the stretch blow molding process, a preform is formed by injection molding or extrusion, the temperature of the preform is controlled to a certain temperature, and the preform is then stretched in a machine direction by means of a stretching rod and in a transverse direction by means of high-pressure air within a blow mold to produce a biaxially stretched blow bottle.
Since PET is low in melt viscosity, the formation of the preform by extrusion involves a problem of drawdown. Therefore, there is generally adopted an injection stretch blow molding process in which a preform is formed from PET by injection molding, and the preform is subjected to stretch blow molding at a temperature not higher than the melting point of PET. This injection stretch blow molding includes a hot parison method that a preform obtained by injection molding is subjected to stretch blow molding in a hot state without completely cooling it, and a cold parison method that a parison obtained by injection molding is cooled to room temperature, and then reheated to a stretching temperature to conduct stretch blow molding.
The biaxially stretched blow bottle obtained by the stretch blow molding of PET is excellent in heat resistance, transparency, gloss, retention of perfume and the like and also relatively good in carbon dioxide gas barrier property and oxygen gas barrier property. However, the biaxially stretched blow bottle of PET is yet insufficient in carbon dioxide gas barrier property and oxygen gas barrier property as a container for alcoholic drinks such as rice wine and beer, carbonated drinks such as soda pop and cola, fruit drinks (including tea, coffee, sports drinks, etc., in addition to fruit juices), medicines, and the like, and thus is required to improve its gas barrier properties from the viewpoint of shelf life (change in taste of contents).
As a method for improving the gas barrier properties of the biaxially stretched blow bottle of PET, there has heretofore been proposed a multi-layer biaxially stretched blow bottle having a 3-layer structure of PET/gas barrier resin/PET or a 5-layer structure of PET/gas barrier resin/PET/gas barrier resin/PET with the gas barrier resin arranged as an intermediate layer, and such bottles have been already marketed. Typical examples of the gas barrier resin include an ethylene-vinyl alcohol copolymer (hereinafter abbreviated “EVOH”) and nylon MXD6. A multi-layer biaxially stretched blow bottle having such a layer structure is generally produced by an injection stretch blow molding process in which PET and a gas barrier resin are co-injection-molded to mold a multi-layer preform, and the multi-layer preform is reheated to a stretching temperature to subject the preform to biaxial stretch blow molding.
Among these gas barrier resins, EVOH is excellent in gas barrier properties under a low humidity, but absorbs moisture under a high humidity to lower its gas barrier properties. Therefore, a multi-layer biaxially stretched blow bottle having an EVOH layer is not suitable for use as a packaging material for water-containing foods and retortable foods.
On the other hand, nylon MXD6 contains no chlorine atom in its molecule but is not very changed in gas barrier properties even under a high humidity. Also, Nylon MXD6 almost consists with PET in injection molding temperature and stretch blow molding temperature, and thus is excellent in co-injection moldability with PET, and so biaxial stretch blow molding can be smoothly performed.
However, nylon MXD6 has a demerit that its gas barrier properties are considerably low compared with other gas barrier resins. In order to sufficiently elongate the shelf life of contents containing carbon dioxide gas or easy to be denatured by oxidation, such as an alcoholic drink, carbonated drink or medicine, nylon MXD6 has had a limit.
Bottles with an oxygen absorbent such as a cobalt salt contained in a nylon MXD6 layer for the purpose of enhancing the gas barrier properties of a multi-layer biaxially stretched blow bottle having a 3-layer structure of PET/nylon MXD6/PET or a 5-layer structure of PET/nylon MXD6/PET/nylon MXD6/PET have heretofore been produced and sold as bottles for beer. However, the inclusion of an inorganic filler such as the cobalt salt in a great amount in nylon MXD6 had possibilities that difficulty would be encountered upon smoothly performing biaxial stretch blow molding, conditions for stretch blow molding would be limited, and the transparency of the resulting bottle would be impaired.
In a biaxially stretched blow bottle composed of 3 layers of PET/barrier resin/PET, has been proposed a gas barrier biaxially stretched blow bottle using, as a barrier resin of an intermediate layer, a gas barrier resin containing a silicate composite obtained by an ion-exchange reaction with a positively charged organic compound in a proportion of 0.1 to 10% by weight (Japanese Patent Application Laid-Open No. 2001-1476, hereinafter referred to as “Reference 1”). This Reference 1 discloses EVOH and nylon MXD6 as gas barrier resins.
When the silicate composite is contained in nylon MXD6 of the intermediate layer, a multi-layer biaxially stretched blow bottle low in oxygen transmission rate can be obtained. However, the method of adding the silicate composite has a possibility that the stretch blow moldability and transparency of the resulting bottle may be adversely affected, in addition to increase in cost. In fact, Reference 1 describes the fact that the preparation of the silicate composite requires to strictly control the amount of a positively charged organic compound ion introduced into a silicate of a lamellar crystal within a fixed range, and no uniform silicate composite may be obtained, or the dispersibility of the resulting silicate composite in the gas barrier resin is deteriorated when the amount introduced is outside the predetermined range.
By the way, when a multi-layer biaxially stretched blow container having a 3-layer structure of PET/nylon MXD6/PET or a 5-layer structure of PET/nylon MXD6/PET/nylon MXD6/PET is produced, stretching conditions such as draw ratio and stretching temperature have been generally caused to consist with the stretching conditions for PET bottles. When a preform of PET is used to produce a biaxially stretched blow bottle, it has been generally common that a draw ratio (stretch ratio) in a machine direction (axial direction) is controlled within a range of about 1.5 times to about 2.0 times, and a draw ratio in a transverse direction (circumferential direction) is controlled within a range of about 4.0 times to about 4.6 times. When the draw ratio in the transverse direction is made high within the above-described respective ranges while retaining an area ratio to about 9 times or lower, the draw ratio in the machine direction has been made low (Handbook of Saturated Polyester Resins, issued by THE NIKKAN KOGYO SHINBUN, LTD., the first issue of the first edition on Dec. 22, 1989, p. 627, Table 12.14).
As to the stretching temperature, in general, a temperature not lower than the glass transition temperature of PET, but not higher than the melting point thereof is widely adopted. However, in fact, the stretching temperature of a preform is often controlled as high as about 95° C. to about 100° C. for the purpose of enhancing orientation crystallizability of PET and preventing whitening of the resulting bottle.
Therefore, the multi-layer biaxially stretched blow bottle having the 3-layer structure of PET/nylon MXD6/PET or the 5-layer structure of PET/nylon MXD6/PET/nylon MXD6/PET has also been generally produced under the above-described stretching conditions.
For example, Examples 1 to 4 of Reference 1 disclose experimental examples where a gas barrier resin with a silicate complex contained in a proportion of 2 to 4% by weight in nylon MXD6 was used produce a biaxially stretched blow bottle having a 3-layer structure of PET/gas barrier resin/PET. According to these examples, it is understood that a multi-layer preform (weight: 29 g, height: 110 mm, diameter: 25 mm) formed by injection molding is reheated to a stretching temperature of 100° C., and the preform is subjected to biaxial stretch blow molding under molding conditions that a draw ratio in a machine direction is of the order of 1.8 to 1.9 times (height of bottle: 200 mm), and a draw ratio in a transverse direction is of the order of a little under 4 times (volume of bottle: 500 ml). Comparative Example 1 of Reference 1 shows that nylon MXD6 containing no silicate complex was used to produce a multi-layer biaxially stretched blow bottle having a 3-layer structure of PET/nylon MXD6/PET under the same stretching conditions as described above.
According to an investigated result by the present inventors, however, it has been found again that when neither a silicate complex nor an inorganic filler for improving oxygen gas barrier property, such as a cobalt salt is added, it is difficult to produce a multi-layer biaxially stretched blow bottle sufficiently improved in oxygen gas barrier property by adopting such conventional stretching conditions.